The present invention relates to switch-mode power supply controllers. More particularly, the present invention relates to a circuit arrangement and method of providing adaptive loop response in switch-mode power supply controllers.
Most modern electronic systems employ switch-mode power supplies due to the high power efficiency and smaller physical size of these types of power supplies. Switch-mode power supplies are also advantageous in that these supplies can provide regulated output voltages having magnitudes higher or lower than the unregulated input supply, and also provide multiple outputs of differing voltage magnitudes.
Many modern electronic systems also employ microprocessors as part of the system design. As microprocessor clock speeds increase, the switching frequency of switch-mode power supplies used in conjunction with microprocessor-based systems must also increase. Thus, the error amplifiers used in the controller circuitry of many switch-mode power supply designs must also have fast response times. Of course, in addition to providing fast response times, the power supply controllers must also provide stable operation at the high clock frequencies utilized by microprocessors.
In most instances, such as steady state or slow load variations, switch-mode power supply controllers provide sufficiently stable operation. However, under certain transient conditions, such as from no-load, or low-load, to a high load condition (or vice-versa), the switch mode power supply closed loop response may be slow to react due to output voltage slew rate limitations of the error amplifier. To compensate for this potential slow response, additional capacitors are placed at the output of the power supply. These additional components both increase system cost and provide a larger power supply footprint.
One method known in the art for increasing the controller response time is to clamp the output of the error amplifier to a predetermined voltage level with respect to a reference (or ground) potential. Since the clamp voltage level is fixed in this arrangement, it must be set above the controller""s oscillator/ramp peak voltage or the maximum output voltage, depending on the design, so as not to limit the maximum duty cycle of the power supply. Typically, this requires clamp voltages in the range of 2 to 8 volts. Because this clamp voltage range is fairly high, the slew time of the error amplifier output over the 2 to 8 volt range may be too high for many high speed microprocessor-based system applications.
Hence, there is a need in the art for a controller circuit design and methodology that increases the response time of the controller""s error amplifier circuit without adversely affecting the controller""s stability.